Soil gas profiles as a tool to characterise active tectonic areas: the Jaut Pass example (Pyrenees, France)

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Abstract

A new method to investigate active tectonic structures, using soil gas composition at faults, suggests relevant information about regional stress conditions which can be obtained rapidly and at relatively low cost. In 1995, we carried out geochemical profiles around the epicentre of the M=5.1 1980 earthquake near Arudy in the French Pyrenees, where the presence of minor fractures is evident in the field, and confirmed using satellite SPOT imagery. Fractures are conduits facilitating fluid migrations in the crust, and are also pathways for the release of deep-seated gases to the atmosphere. In order to investigate the implication of these fractures in the present deformation, i.e. if they are connected to the Hercynian substratum at a depth of about 1800 m, soil gases were measured along four traverses crossing the observed structures. Gases determined were 222Rn, CO2 and 4He, each of them for their characteristic source: 222Rn has essentially a shallow origin due to its short half-life, whereas CO2 is the major soil gas component with a mainly surficial biogenic source. However, there could additionally be CO2 from crustal or mantle degassing, which would also be the principal sources of He. Data analysis clearly reveals anomalous values for each gas at specific positions along the traverses. Two sets of fractures corresponding to different observed trends are distinguished: the one characterised by He anomalies accompanied by other gases, and the second with few identified He anomalies. The agreement between the geochemical data and the field observations leads us to propose a deformation model for the area studied, analogous to a pull-apart system located in a right lateral shear zone.

Introduction

Crustal discontinuities, such as fractures and faults of various dimensions, facilitate degassing flux from the Earth to the hydrosphere and the atmosphere. One of the most striking features of such volatile leaking is the widespread distribution of gas-rich spring waters over tectonically active areas. Highest gas fluxes occur in two different environments: recent and present volcanic areas where gas seepage is located both at central vents and often in large distal areas, and seismically active zones where evidence of preferential degassing near active faults is common. In the seismic case, degassing has been shown to occur mainly as advective fluxes through soils of fractured areas, and/or as free non-mixed gas phase from thermo-mineral springs due to pressure drop during ascent of the fluid to the surface [1]. These water and gas discharges over seismically active faults correspond to a long-term, permanent (with respect to earthquake recurrence times) phenomenon which indicates that active faults are characterised by a high permeability, and act as preferential conduits in the crust [1]. Because fluid transfer in the crust is strongly promoted by fractures, high geochemical contrasts are expected in faulted zones [2]. The main degassing species from the earth is carbon dioxide, giving soda spring provinces for areas of heat flow anomalies, and the second is nitrogen, giving nitrogen hydrothermal spring provinces. Both can be characterised from two of their minor gas components, helium and radon. However, degassing at active faults is also a valid feature for many other terrestrially generated gases and highly volatile metals [1], [3].

Even in a restricted fractured area, gas fluxes can display contrasting patterns. This feature can be attributed, not only to the diverse sources and to the physical/chemical characteristics of the different gases, but also to the complexity of the structural, hydrologic and lithologic patterns of the studied area [1], [2]. It has been shown that the contrasting permeability in fault gouges and intensely sheared zones generate complex geochemical patterns in soil atmospheres [4], [5]. This characteristic has already been used to search for active faults, mainly using radon emanation, but sometimes also using H2, He and CO2 distribution in soils. On the other hand, geomorphology also supplies first order information about the fracture network at the surface, which can be mapped from satellite imagery interpretation and field observations. However, up to now, very few studies have used both geochemical and morphotectonic arguments for seismotectonic characterisation.

Herein this new approach originates from the interpretation of soil gas concentrations to propose a seismotectonic interpretation for a complex fractured area. This area experienced a M=5.1 earthquake 20 years ago, and experiences frequent seismic events with magnitudes ranging between 1 and 4.5.

Section snippets

Seismotectonic settings

The western part of the Pyrenees is the most seismically active area of the range as observed from historical and instrumental seismicity [6], [7]. During the last 35 years, two of the three M>5 earthquakes which occurred in the French Pyrenees took place in this zone: the 13 September 1967 Arette earthquake with a magnitude of 5.7, and the 29 February 1980 Arudy earthquake of magnitude 5.1. The former, with two different determined focal mechanisms [8], [9], is poorly constrained due to the

Principle

Many gaseous compounds have been well documented in subsurface soil air: atmospheric gases, carbon- and nitrogen-containing gases, rare gases, hydrogen, and volatile heavy metals. As complex processes such as mantle degassing, crustal radiogenic production, rock alteration, biogenic activity and atmospheric dilution, at various depths, are involved in the generation of these gases prior to them reaching the ground surface, and because several phenomena such as mixing, contamination, chemical

The Jaut Pass geochemical profiles

Several areas of the Jaut Pass were selected for gas geochemical traverses on the basis of morphological arguments (Fig. 2a) in order to correlate with seismotectonic features.

Traverse J1 is 1655 m long, parallel to the E–W cliff that cuts the main N140/N150°E and N0/N20°E fractures which appeared on both the field and satellite images (Fig. 2b). Traverse J2 is shorter (430 m long), slightly eastward from J1, and was operated with a 5 m sampling interval to control the geochemical anomalies

Seismotectonic interpretation

Occurrences of earth degassing, demonstrated by helium anomalies in soils over the N140/N150°E and N30/N50°E faults, imply that they have a compatible orientation relative to the local stress field, i.e. they are characterised by open channels and therefore convenient for gas transfer from depth. In contrast, the N0/N20°E fractures at the Jaut Pass are ‘closed’, because they do not allow gas migration from depth to the surface since they are not correctly oriented in the actual regional stress

Conclusions

Soil CO2, Rn and He gas patterns, combined with morphological and geological observations, can supply useful constraints for deformation models in continental environments. In particular, the previous recognition of possible tectonic structures allows targeting of geochemical profiles in order to ‘rapidly’ obtain measurements of gas anomalies over the intersecting structures. Moreover, the chemical compounds measured were selected for the source information they are expected to supply:

Acknowledgements

The authors are grateful to Michel Blanc for providing the technical capability for the field work, and Christian Ponsolles for his well-organised field assistance. Thanks are also due to Richard Nicholson for undertaking the correction of the English text, J.-C. Sabroux and an anonymous reviewer for their constructive criticisms of the manuscript. The study was conducted with financial support from PNRN/INSU (Programme National Risques Naturels) and Université Paul Sabatier (Toulouse).[AC]

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